This invention relates to a method and means for catalytically reducing the various amounts of pollutants found in the exhaust gases emanating from the internal combustion engine.
Presently, much effort is being directed toward the removal of carbon monoxide and unburned hydrocarbons by thermal or catalytic oxidation, and to the diminution or elimination of nitrogen by catalytic reduction.
With regard to the elimination or diminution of the various combustibles found in the exhaust gases emanating from the internal combustion engine, present U.S. Governmental standards require that by 1976 the hydrocarbon content of the exhaust gases be reduced to at least 0.41 grams per mile and that the carbon monoxide content be reduced to at least 3.4 grams per mile. In addition, these standards also require that the nitrogen oxide (NO.sub.x) content of the exhaust gas be reduced to less than 0.4 grams per mile by 1977.
One system which has been suggested for accomplishing such a reduction of pollutants is the so-called dual bed catalyst system. In this system, two catalysts are utilized. One catalyst is designed to reduce nitrogen oxide emissions to less harmful substances while another catalyst is employed to aid in the oxidation of carbon monoxide and the various hydrocarbon gases present in the exhaust gases.
The catalyst which is most often suggested for use in oxidizing carbon monoxide and hydrocarbon gases is composed of a ceramic substrate, e.g., aluminum oxide, having deposited on the surface thereof a platinum catalyst.
With regard to the nitrogen oxide (NO.sub.x) reducing catalyst, various structures have been suggested. One such structure consists of a metallic substrate having adherently bonded to the surface thereof a NO.sub.x reducing catalyst. Such a type of catalyst is more fully described in copending application Ser. No. 207,338, which is assigned to the assignee of the present application.
As used herein both the terms "nitrogen oxide" and "NO.sub.x " refer to the various oxides of nitrogen found in the exhaust gases of the internal combustion engine.
In the operation of a dual bed catalyst system which employs these general types of catalysts, the exhaust gases are brought into contact with the NO.sub.x reducing catalyst and then brought into contact with the oxidation catalyst to effect the oxidation of carbon monoxide and/or the various unburned hydrocarbons which may be present.
While such a dual bed catalyst system is highly successful in reducing the amount of undesirable components found in the exhaust gases emanating from the internal combustion engine, difficulties are encountered with regard to the longevity of this catalyst system. In the main, such difficulties are associated with the fact that the NO.sub.x reducing catalyst begins to deteriorate rather rapidly, thereby limiting the life of the dual catalyst system.
Recently it has been discovered that the life of a NO.sub.x reducing catalyst of the foregoing type can be significantly improved by bringing the exhaust gases into contact with an oxidation catalyst to substantially remove the unreacted oxygen therefrom and then subjecting the so-treated exhaust gas to the action of the NO.sub.x reducing catalyst. While this discovery resulted in a NO.sub.x reducing catalyst which evidenced increased durability, a secondary problem was experienced. Basically, during the preliminary treatment with the oxidation catalyst, nitrogen and hydrogen present in the exhaust gases reacted to form ammonia. The ammonia then passed through the NO.sub.x catalyst and when brought into contact with the second oxidation catalyst was oxidized to form NO.sub.x. Obviously, this system resulted in a final product very high in NO.sub.x content. Subsequently, it was quite unexpectedly discovered that the problem of ammonia generation could be avoided by employing an oxidation catalyst which comprises a metallic substrate having a layer of electrically, nonconducting metal oxide on the surface thereof and a platinum group metal adherently deposited on at least part of the surface of the nonconducting metal oxide.